Optimal mean airway pressure during high-frequency oscillation - Predictedby the pressure-volume curve

Background: A number of groups have recommended setting positive end-expira tory pressure during conventional mechanical ventilation in adults at 2 cm H2O above the lower corner pressure (P-CL) of the inspiratory pressure-volu me (P-V) curve of the respiratory system. No equivalent recommendations for the setting of the mean airway pressure (P-aw) during high-frequency oscil lation (HFO) exist. The authors questioned if the P-aw resulting in the bes t oxygenation without hemodynamic compromise during HFO is related to the s tatic P-V curve in a large animal model of acute respiratory distress syndr ome. Methods: Saline lung lavage was performed in seven sheep (28 +/- 5 kg, mean +/- SD) until the arterial oxygen partial pressure/fraction of inspired ox ygen ratio decreased to 85 +/- 27 mmHg at a positive end-expiratory pressur e of 5 cm H2O (initial injury). The P-CL (20 +/- 1 cm H2O) on the inflation Limb and the point of maximum curvature change (PMC; 26 +/- 1 cm H2O) on t he deflation limb of the static P-V curve were determined. The sheep were s ubjected to four 1-h cycles of HFO at different levels of P-aw (P-CL + 2, 6, + 10, + 14 cm H2O), applied in random order. Each cycle was preceded by a recruitment maneuver at a sustained P-aw of 50 cm H2O for 60 s. Results: High-frequency oscillation with a P-aw of 6 cm H2O above P-CL (P-C L + 6) resulted in a significant improvement in oxygenation (P < 0.01 vs. i nitial injury). No further improvement in oxygenation was observed with hig her P-aw, but cardiac output decreased, pulmonary vascular resistance incre ased, and oxygen delivery decreased at P-aw greater than P-CL + 6. The PMC on the deflation limb of the P-V curve was equal to the P-CL + 6 (r = 0.77, P < 0.05). Conclusion: In this model of acute respiratory distress syndrome, optimal P -aw during HFO is equal to P-CL + 6, which correlates with the PMC.